1. Field of the Invention
The present invention relates to an image forming apparatus which has an electrostatic image developing toner to develop an electrostatic image in an image forming process, such as electronic photography, electrostatic recording, or electrostatic printing.
2. Related Background Art
As described in U.S. Pat. No. 2,297,691 and Japanese Patent Publications No. 42-23910 and No. 43-24748, many processes of electrophotography are known. These processes typically consist of the following steps: (1) the surface of an electrostatic image bearing member using a photoconductive substance is uniformly charged by various means, (2) an electrostatic image is formed on the image bearing member, (3) the electrostatic image is developed using a toner, (4) the toner image is transferred onto a transfer material, such as paper, as needed, (5) the image is fixed by heating, pressurizing, or both of them or with solvent vapor or the like to obtain a print, and (6) by various methods, the image bearing member is cleaned of toner which is left untransferred onto the electrostatic image bearing member. These steps are repeated.
Methods for removing residual toner from an electrostatic image bearing member, which include a blade cleaning method, a fur brush cleaning method, and a magnetic brush cleaning method, commonly bring a cleaning member into contact with an electrostatic image bearing member.
Many contact charging methods have recently been proposed which bring a charging member into contact with the surface of an electrostatic image bearing member to apply a voltage produced by superposing a DC voltage and an AC voltage onto the image bearing member. These methods allow a lower voltage to be used, compared with the conventional corona charging method. Contact charging methods also have the advantage of producing a small amount of ozone. For example, as shown in FIG. 4, contact charging methods drive a charging roller 2, or a charging member, which is brought into contact with an electrostatic image bearing member (a photosensitive drum) I, to apply a voltage (Vac+Vdc) produced by superposing an AC voltage and a DC voltage to the charging roller 2 so that the photosensitive drum 1 can be uniformly charged.
As understood from the foregoing, the charging roller 2 must be kept electrically conductive. A conventional charging roller is a core around which an electrically conductive elastic member is formed by dispersing carbon in elastic rubber, such as EPDM and NBR.
In recent years, expanded rubber has been often used to reduce vibration noise which is produced because of an AC voltage applied for charging. A transfer roller, or a core covered with elastic rubber, such as electrically conductive expanded EPDM, is used for image transfer. Because contact charging methods press toner against an electrostatic image bearing member, such as a photosensitive member, toner packing occurs, thus causing a phenomenon called xe2x80x9chollow imagesxe2x80x9d: the middle of an image is not transferred but left on an electrostatic image bearing member.
To solve this problem, the pressure under which the transfer roller is pressed down is reduced, or the transfer material speed and electrostatic image bearing member speed are made to differ from each other to scrape the middle of an image using a shear force and transfer the scraped part onto a transfer material.
However, if transfer roller pressure is reduced, an image tends to be blurred due to shock which is caused by a transfer material passing through other transport rollers. On the other hand, if the electrostatic image bearing member speed and transfer material speed differ from each other, the image magnification must be adjusted by controlling the circumferential speed of the transfer roller, which exhibits higher transport performance than the electrostatic image bearing member. The transfer roller easily wears because of the circumferential speed difference. Making many prints causes the outer diameter of the transfer roller to decrease, thus reducing its circumferential speed, so that the transfer roller becomes less effective in preventing xe2x80x9chollow images.xe2x80x9d
To meet a growing demand from users for energy saving, it is better that a toner is packed in paper fibers, or toners are packed in each other so that fixing performance can be improved, that is, heat is transferred more effectively during fixing. Toners (or particles) tightly stuck together advantageously prevent a phenomenon called xe2x80x9csmeared images with trailing edgesxe2x80x9d: the scattering on a toner image occurs toward the upstream direction of transport due to steam produced by fixing. However, pressure cannot be applied due to the above-described problem of transferred-image quality.
An object of the present invention is to provide an image forming apparatus which solves the above-described problems.
Another object of the present invention is to provide an image forming apparatus which forms images free from xe2x80x9chollow imagesxe2x80x9d and blurred images for a long period of time and prevents image deterioration after image transfer.
Still another object of the present invention is to provide an image forming apparatus which fixes an image using less energy and prevents smeared images with trailing edges during fixing.
The present invention provides an image forming apparatus, comprising:
(i) an electrostatic image bearing member for bearing an electrostatic image;
(ii) developing means having a toner for forming a toner image by developing an electrostatic image carried on the electrostatic image bearing member; and
(iii) transferring means for transferring the toner image from the electrostatic image bearing member onto a transfer material, wherein
particles constituting the toner are 4 to 12 xcexcm in weight average diameter, and in a circularity distribution for particles 3 xcexcm or more in circle-equivalent diameter which are included in the toner, particles having a circularity a of 0.90 or more account for 90% by number or more, the circularity a being given by the following formula:       Circularity    ⁢          xe2x80x83        ⁢    a    =            L      0        L  
xe2x80x83where
L0: The circumference of a circle which has the same projected area as a particle image
L: The perimeter of the particle image, and
the transferring means has a transfer roller which is in contact with the surface of the electrostatic image bearing member and rotates as the electrostatic image bearing member moves.